Cell fate after DNA damage is determined by the cellular DNA damage response (DDR) mechanism that encompasses DNA repair, cell cycle checkpoints, and programmed cell death. Although extensive efforts were dedicated to revealing the activation network of the DDR, little is known about how the cell deactivates the DDR and resumes cell cycle progression, a process termed checkpoint recovery. Importantly, emerging evidence suggests that cancer cells may utilize the recovery mechanism to bypass the DDR during tumorigenesis, and that checkpoint recovery from radio- or chemotherapy causes chemoresistance and tumor recurrence, suggesting checkpoint recovery as an ideal target to improve cancer therapy and patient survival. The long-term goal of this project is to delineate the mechanism of checkpoint recovery and to investigate the implication of checkpoint recovery for cancer progression and therapeutics. We recently characterized the first in vitro system to study checkpoint recovery, and used this system to identify Greatwall (Gwl) kinase as an essential regulator of checkpoint recovery. Interestingly, our preliminary studies discovered novel phosphatases-dependent regulation of checkpoint recovery kinases; our results suggested coordinated activation of Gwl and Plk1, and characterized Gwl phosphorylation and interaction as early events of checkpoint recovery. Moreover, preliminary studies in cancer cell lines and mouse tumor models suggested Gwl as an effective target to enhance chemotherapy and block tumor recurrence. In this proposal we will address fundamental questions regarding the regulatory mechanism of Gwl in checkpoint recovery and its role in cancer therapy and tumor recurrence. Aim 1 will investigate how Gwl and Plk1 are regulated by DNA damage. Our study will take advantage of the cell-free system and a site-specific phosphatase assay to investigate DNA damage-induced, and phosphatase-dependent regulation of Gwl and Plk1. Aim 2 will ask how Gwl and Plk1 re-activate from checkpoint arrest. We will characterize novel Gwl/Plk1 interaction and phosphorylation as potential molecular switches that initiate recovery. In Aim 3, we will characterize how Gwl up-regulation promotes cell recovery from cisplatin treatment and thereby facilitating tumor recurrence. We will then directly evaluate new therapeutic ideas in which targeting Gwl or its initial activation during checkpoint recovery leads to more effective and specific cancer therapy.